This invention relates to a system for extracting information from a carrier wave and relates generally to the method and apparatus described in U.S. Pat. Nos. 4,106,007 and 4,218,655, the disclosures of which are incorporated herein by reference. As described in those patents, it is known that a modulation voltage can be superimposed on a power system voltage, at specified locations on the power system voltage such as a zero crossing, to cause wave shape perturbations in the carrier wave. In the embodiment described hereinafter, the carrier wave is the voltage wave of an electrical power distribution system.
Communication over electric power distribution lines is useful for signaling, meter reading, and load control, among other uses. However, communication over an electric distribution system is a complex undertaking. Each customer service constitutes a branch in the distribution feeder, and the branching is so extensive that it is impractical to provide filter and by-pass circuitry at each branch point. The distribution system is not an attractive medium for conventional communications due to the attenuation and dispersion of the signals and because noise levels tend to be high. To overcome the high noise levels, it is generally necessary to use narrow band filtering, error-detecting and error-correcting codes, and relatively high signal power levels at low bit rates.
The aforementioned problems arise in two areas. The first concerns transmitting information from the central source in the direction of energy flow to the individual customer premises. This transmission of information in the direction of energy flow is referred to as "outbound" signaling. Functions such as automatic meter reading and various alarm systems, however, require that information passes not only from a single source to the end user, but also from the end user back to the central station. This transmission of information in the direction opposite to that of the energy flow is referred to herein as "inbound" signaling.
In the system described in the aforementioned patents, each binary digit (a binary "1" or a binary "0") is made up of four current pulse modulations located at preselected zero crossings of the electrical distribution network voltage waveform. These current pulses are located within eight zero crossings (four complete cycles) of the waveform and the current pulse patterns for "ls" and "0s" are complementary.
By using different pulse patterns to define binary "1s" and "0s," it is possible to define a number of separate channels over which information can be transmitted in each eight half-cycle segment of the waveform. Heretofore, however, it has been difficult to identify sets or groups of such channels which were independent or non-interfering, i.e., channels with the property that the presence of a signal on one channel of the set did not interfere with the detection and identification or a signal on another channel of the set.
Furthermore, it has proven extremely difficult to determine the maximum number of non-interfering channels for any given system configuration and to identify sets of channels containing the maximum number of non-interfering channels.
SUMMARY OF THE INVENTION
One of the objects of this invention is to provide a method for improving the transmission rate of inbound information in an electric distribution system.
Another object of this invention is to provide a system with the maximum possible number of independent, non-interfering channels.
Another object is to provide a generalized yet effective method for determining the maximum number of independent, non-interfering channels in a given system.
A four object is to provide a method for identifying groups of channels which have the maximum possible number of independent, non-interfering channels.
Other objects and features of this invention will be in part apparent and in part pointed out hereafter.
Briefly, in a first aspect a system of the present invention is designed to substantially simultaneously transmit signals composed of binary digits inbound over an electricity distribution network and detect said simultaneously transmitted inbound signals. Each binary digit transmitted is composed of a first predetermined number of current pulses superimposed on preselected zero crossings contained within the first predetermined number of cycles of the voltage waveform of the electricity distribution network. The system includes designating a second predetermined number of channels (the second predetermined number being at least fifty percent greater than the first predetermined number), each channel being defined by specifying the set of zero crossings which define a binary "1" for that channel and the set of zero crossings which define a binary "0" for that channel. The sets of zero crossings for each channel are mutually exclusive and each contains the first predetermined number of zero crossings. Each set of zero crossings is unique with respect to all the other sets for the various channels. The system further includes sending binary digits inbound simultaneously over the second predetermined number of channels. For the first predetermined number of cycles of the voltage waveform, the magnitudes of the current pulses at all the zero crossings are measured. Also for each channel, a detection algorithm is applied to the measured magnitudes of the current pulses for all the zero crossings. Each detection algorithm is unique with respect to all the other detection algorithms, the detection algorithms being selected so that all the designated channels are non-interfering even though all use exactly the same zero crossings of the voltage waveform. A binary "1" is detected for any particular channel if the application of the detection algorithm for that channel to the measured magnitudes of the current pulses for all the zero crossings equals a third predetermined number. A binary "0" is detected for any particular channel if the application of the detection algorithm for that channel to the measured magnitudes of the current pulses for all the zero crossings equals the negative of the third predetermined number. And the absence of a binary digit for any particular channel is detected if the application of the detection algorithm for that channel to the measured magnitudes of the current pulses for all the zero crossings equals a number other than the third predetermined number or its negative.
In a second aspect a method of the present invention is directed to identifying non-interfering inbound channels in a system for substantially simultaneously transmitting signals composed of binary digits inbound over an electricity distribution network and to detecting said simultaneously transmitted inbound signals. Each binary digit is composed of a first predetermined number of current pulses superimposed on preselected zero crossings contained within a second predetermined numbers of cycles of the voltage waveform of the electricity distribution network. Each channel is defined by specifying the set of zero crossings which define a binary "1" for that channel and the set of zero crossings which define a binary "0" for that channel. The sets of zero crossings for each channel are mutually exclusive and each contains the first predetermined number of zero crossings. The method includes the steps of identifying the potential channels by organizing the zero crossings in the second predetermined number of cycles of the electricity distribution network waveform into sets, each set containing the first predetermined number of separate zero crossings. Each pair of mutually exclusive sets constitutes a potential channel. First and second pure pulse patterns are defined for each potential channel, each pure pulse pattern being a set of "j" numbers, where "j" is the number of zero crossings in the second predetermined number of cycles of the waveform. One of the pure pulse patterns for each channel represents the presence of a binary "1" in that channel with no signal present in any other channel and the other pure pulse pattern for each channel represents the presence of a binary "0" in that channel with no signal present in any other channel. Each pure pulse pattern is unique. Detection matrices are selected for the channels, each detection matrix being unique with respect to the other detection matrices. Each detection matrix when multiplied by a pure pulse pattern results in a first preselected number when the pure pulse pattern represents a binary "1" in the channel corresponding to that detection matrix, results in a second preselected number when the pure pulse pattern represents a binary "0" in the channel corresponding to that detection matrix, and results in a third preselected number when no signal is present. A composite detection matrix is formed from the detection matrixes for all the potential channels, each row of the composite detection matrix being the detection matrix for the channel corresponding to the row number of that row. A composite pulse pattern matrix is formed from the first pure pulse patterns for each channel, each column of the composite pulse pattern matrix being the first pure pulse pattern for the channel corresponding to the column number of that column. The composite detection matrix is multiplied by the composite pulse pattern matrix to form a channel analysis matrix, each element of the channel analysis matrix being a number resulting from the application of the detection matrix for the channel corresponding to the column number of the element to the first pure pulse pattern for the channel corresponding to the row number of the element. The method also involves categorizing as qualifying elements only those elements of the channel analysis matrix whose numerical value corresponds to the first preselected number or the third preselected number. Independent channels are identified by determining which sets of rows of the channel analysis matrix have in common qualifying elements in the columns corresponding to all the row numbers of the set. The row numbers of each such set represent a set of independent channels.